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Discussion about software development for the old-school Gameboys, ranging from the "Gray brick" to Gameboy Color
(Launched in 2008)

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#1 2019-09-11 12:17:23

Ardis
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From: USA
Registered: 2019-06-06
Posts: 16
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Looking for help designing an MBC3 flashcart with RTC

Some time ago, I found this nifty little tutorial and thought it would be fun to try setting it up in one of my cartridges at a later date. It adds a rechargeable battery into the cartridge and a separate charging circuit with a micro-USB port for recharging the battery.
https://imgur.com/a/Sn0U7

However, it does this by having a second circuit board and battery stuffed into the shell. Being something of a neat freak, I wanted a more elegant solution. I set out to design a 2MB flashcart with a working RTC, non-volatile RAM for saves and a charging circuit built into the board for a rechargeable RTC battery.

The problem is that, I have absolutely no idea what I am doing when it comes to picking the right chips for this application. I see a lot of specifications on the chips and don't know if they're relevant in the situation of an MBC cart. I know what will work for a 32KB flashcart and even intend to do a test run of my designed cartridge soon. I'm aware that there are affordable RTC MBC3 flashcarts out there, but the only ones I've seen have been built by dismantling existing cartridges. Not something I really condone. I want to make new cartridges, better ones if I can.

I'm not as worried about the flash chip as I've seen what others are using on their flashcarts and can just use those same chips, but I would like to make sure that I'm understanding the requirements in case I want to go with a more affordable chip later on. Does the organization of data on either the flash or RAM chips matter? Or will any 5V chip with enough pins and a capacity higher than 2 megabytes/16 megabits work? (Looking at other flashcarts seems to confirm it, but I'm not as certain with the RAM situation.)


The one that has me more worried is the MBC3. I'm not sure how to go about making an MBC chip in the first place, let alone an MBC3 which relies on an external oscillator for its clock functions. Are there any resources I can be pointed to? Google hasn't proven very helpful to me on this, instead pointing me to MBC5 which doesn't have the RTC functions I am looking for.

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#2 2019-09-11 21:41:07

WeaselBomb
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Registered: 2018-03-06
Posts: 27
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Re: Looking for help designing an MBC3 flashcart with RTC

I think we are after the same thing. I was looking for a non-volatile solution too, and found that exact rechargeable battery mod. I too didn't think it was precise and elegant enough to be practical. Then I found BitBuilt, where a guy designed an MBC5 flash cart with FRAM. I decided to buy one, but when I finally got it (9 months later btw), I realized it didn't work with certain games, namely Super Mario Land 2. More importantly, RTC wasn't supported. So, since no options existed, I decided to make my own.

It supports 2 MB Flash, 32 KB FRAM, a CR2025 coin cell, and a genuine MBC3 chip. If you want to use it for anything, you can download the gerber, pcb file, and schematic from RetroReboot.net. The full parts list should be listed there too.

I understand when you say you don't want to dismantle an existing cartridge. I don't want to dismantle them either, and am currently trying to work out a design using a CPLD that will hopefully support MBC 1-5 and RTC. My progress is actually in another thread on this forum, "MBC5 in WinCUPL" if you want to take a look. There isn't much there yet, but it's slowly creeping along.

Ardis wrote:

Does the organization of data on either the flash or RAM chips matter? Or will any 5V chip with enough pins and a capacity higher than 2 megabytes/16 megabits work?

You can use any memory capacity, as long as its big enough to hold the program you want. Just be careful with unused address pins. Make sure you ground them, or they will "float", resulting in the wrong/unexpected address being read/written.
For the organization, I always refer to a chip that I know already works, like the AM29F016B, for instance. It's divided as 2M x 8, so whatever flash chip I pick, I make sure it is also x8. For RAM, I like the FM18W08. You can use x16, but you will have a lot of extra unused pins I think.

P.S. I really hope this project works out for you. I'm very new to electronics, so a project that implements custom RTC is quite a challenge. I would love to see a project like this succeed, whether it's mine or yours, or someone else's. Who knows, we may even figure it out together!

Last edited by WeaselBomb (2019-09-11 23:14:48)

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#3 2019-09-11 23:22:38

Ardis
Member
From: USA
Registered: 2019-06-06
Posts: 16
Website

Re: Looking for help designing an MBC3 flashcart with RTC

I didn't look at your thread initially because I saw MBC5 in the title and didn't realize you were also looking to do MBC3+RTC stuff. I will also admit that I am absolutely new to this stuff. I just got EAGLE a few months ago because I wanted to design my own 32KB cartridge for fun (and to see if it would be cheaper to order a run of them pre-assembled from a PCB manufacturer than it would be to order complete flashcarts.)

I only started looking at designing an RTC capable cartridge after playing a Pokemon Crystal ROM hack that I really enjoyed and wanted to see if I could flash it, and the save, to a physical cartridge so I could play it on live hardware instead of an emulator (without having to sacrifice an existing game cartridge.)

Since it looks like the answers I need will be found on your thread as it progresses, I'll just start watching that and hope for the best for your project.

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#4 2019-09-11 23:50:00

WeaselBomb
Member
Registered: 2018-03-06
Posts: 27
Website

Re: Looking for help designing an MBC3 flashcart with RTC

Yeah, I wish I could change the title on that to something more appropriate...I kind of just kept piling onto that same thread instead of making a new one.
Well, if you change your mind about sacrificing a game cartridge, you're more than welcome to download those gerber files. I've played Pokemon Prism on it just fine, if that's the hack you're thinking of. I understand if you don't want to do that, though. I've only dismantled one, a copy of "Mary Kate and Ashley: Pocket Planner", and I'd rather not do another. I'd never do that to a Pokemon cart!

I'm not sure if my project will ever succeed or not, so don't hold your breath on that one. RTC is the only thing left to figure out, but that's a challenge all on its own. If you want to do your own cart, I'd be more than happy to offer any help I can, and I'm sure other users around here (who probably know a lot more than me) would too.

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#5 2019-09-12 07:55:16

Ardis
Member
From: USA
Registered: 2019-06-06
Posts: 16
Website

Re: Looking for help designing an MBC3 flashcart with RTC

I did play Pokemon Prism as well, but I was referring to Crystal Clear as it maintains compatibility with Pokemon Stadium 2 and I thought it would be fun to use that feature.

Your project is more likely to succeed than mine. I know almost nothing about how the MBC works in the first place, let alone the RTC other than it using the 32.768hz oscillator to advance a clock. It sounds like you've already figured out the addresses it uses in the save memory. Though something I would recommend checking is to see if it uses those same addresses in other games that use the RTC. I was honestly hoping someone already had a working MBC3+RTC figured out that I could base my cartridge off of. Just try to rearrange things, find smaller memory chips to make room for the charging circuit, etc.

If I were to sacrifice an RTC cartridge, I would probably ensure that it is something that no one really cares about. That would involve doing a lot of research, though. Something even collectors wouldn't touch. Or just a cartridge that isn't working due to a bad ROM chip or something. I suppose finding a bootleg with a working RTC would also work.

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